1. Technical Field of the Invention
The present invention relates to a failure detection apparatus for detecting a failure of a particulate filter that collects particulate matter contained in the exhaust gas of an internal combustion engine.
2. Related Art
In order to reduce soot discharged from an combustion engine such as a diesel engine in the related art, a diesel particulate filter (DPF), which collects granular matter, i.e., so-called particulate matter (PM) constituting the soot, can be disposed in an exhaust passage of the engine. Such a DPF is ensured to be repeatedly usable by periodically performing regeneration process in which the PM accumulated in the DPF are burned and removed.
However, the regeneration process may cause the temperature of the DPF to be excessively elevated. In this case, the DPF may be melted or broken due to the excessively elevated temperature (the DPF may fail). When the DPF is in a failure state, the PM that passes through the DPF is increased, and thus the DPF is unlikely to satisfy the emission limits.
Recently, development of a failure detection apparatus that detects a failure of the DPF is desired, because of request of On-Board Diagnostics (OBD) performed by a computer mounted on a vehicle. As such an apparatus, a failure detection apparatus using an electrode-type PM sensor (see JP-A-2009-144577 and JP-A-S62-035252) that detects an amount of PM is suggested (see JP-A-2009-144577).
An electrode-type PM sensor is configured by providing a pair of electrodes on an insulating base. The PM sensor is located in an exhaust path, and upon a use of the PM sensor, a voltage is applied across the pair of electrodes.
The PM contained in an exhaust gas is deposited on the electrical insulating material between the pair of electrodes. The PM is composed of carbon particles and has electrical conductivity. Accordingly, increase in the amount of deposited PM allows current to pass across the pair of electrodes (i.e. the PM sensor is energized). The value of the current is equivalent to the amount of deposited PM, i.e. equivalent to the amount of PM contained in the exhaust gas. The PM sensor reads the value of current (resistance between the electrodes, which is equivalent to the value of current) to thereby detect the amount of PM.
If the DPF is in a failure state, the amount of PM passing through the DPF will be increased and the resistance across the pair of electrodes of the PM sensor will be decreased. For this, the failure detection apparatus disclosed in JP-A-2009-144577 is configured to detect the resistance of the PM sensor (output of the PM sensor). In this device, when the resistance becomes smaller than a reference resistance, a failure is determined as having occurred in the DPF. In other words, the failure detection apparatus disclosed in JP-A-2009-144577 detects a failure of the DPF based on the absolute value of the output from the PM sensor.
However, the PM that has been deposited on the insulating material between the electrodes of the PM sensor will have an electrical resistance which considerably varies depending on temperature. Specifically, the failure detection apparatus disclosed in JP-A-2009-144577, for example, may determine the DPF as being in a failure state irrespective of the DPF's being in a normal state, or determine the DPF as being in a normal state irrespective of the DPF's being in a failure state. Thus, the failure of the DPF is not accurately detected with such a failure detection apparatus as disclosed in JP-A-2009-144577.